Generally, a solar thermal system is configured to include a heat-collecting unit, a heat storage unit, and a use unit and is a system that stores the heat collected in the heat-collecting unit in the heat storage unit through a heat exchanger and allows the use unit to use the stored heat as a heating and hot water supply heat source.
There are problems in that the existing solar thermal system is expensive, in case of heating, the use efficiency of the system is insignificant as 40 to 50%, a failure of the system frequently occurs due to overheating, and the like.
That is, when a temperature of heating return water passing through a heating load is higher than that of a lower portion of a heat storage tank, temperature stratification in the heat storage tank is scattered, such that the stored heat value is not sufficiently used as a heating source. When a design of a heating system is wrong, an auxiliary boiler may heat water in the heat storage tank under the heating condition in which preheating is performed by solar heat and heat exchange with a heat collector is performed at a lower portion of the heat storage tank to sequentially heat water. Therefore, a load response speed is actually slow when the heat source is always required like winter season, such that the overall system efficiency may be degraded.
Further, the existing solar thermal system has problems in that there is no warning system even when leakage occurs in a heat medium circulation line, a consumer directly supplements water as much as a naturally decreased amount, and the stored water temperature rises to 80° C. or higher in summer season, such that it is likely to have a burn at the time of using hot water.
Further, when the system is overheated, the collected heat is discharged to an AC power heat radiator installed in the heat-collecting unit, such that the external power usage may be increased.